1. Field of the Invention
The present invention relates to a projection display device for enlarging and projecting an image on an imager onto a projection plane.
2. Disclosure of Related Art
Currently, a projection display device (hereinafter, called as a “projector”) for enlarging and projecting an image on an imager (such as a liquid crystal panel) onto a projection plane (such as a projection screen) has been commercialized and widely spread. In the projector of the above type, for instance, three transmissive liquid crystal panels are used as imagers. Light of red, green and blue that has been modulated by the respective corresponding panels is combined by a dichroic prism, and the combined light is projected onto the projection screen by a projecting optical system.
In the above arrangement, normally, the light of the respective colors transmitted through the dichroic prism is respectively linearly polarized light which is oscillated in only one direction. If, however, the light of the respective colors is incident onto the projecting optical system in the above state, there occurs the following drawback that the intensity distribution of light transmitted through the projecting optical system (i.e. a lens group) may be made non-uniform.
Specifically, the transmittance of light with respect to a lens is increased, as the light is incident onto an incident surface of the lens in a state of P-polarized light; and on the other hand, the transmittance is decreased, as the polarization state approaches a state of S-polarized light. P-polarized light is linearly polarized light that is oscillated in a plane (hereinafter, called as a “normal plane”) including an incident direction of light, and a normal to an incident point. S-polarized light is linearly polarized light that is oscillated in a plane perpendicularly intersecting with the normal plane.
In the above arrangement, in the case where the polarization direction of light of the respective colors is aligned only in one direction as described above, the polarization direction of light with respect to the normal plane may be changed, as a position of the lens in a circumferential direction thereof is changed. As a result, the light transmittance may be changed depending on the position of the lens in the circumferential direction. Thereby, the intensity distribution of light transmitted through the projecting optical system (i.e. a lens group) may be made non-uniform resulting from non-uniformity in the light transmittance.
The above drawback occurs similarly in the case where light is incident onto a reflection surface. In the case where light is incident onto a reflection surface as S-polarized light, the light reflectance is increased. On the other hand, in the case where light is incident onto the reflection surface as P-polarized light, the light reflectance is decreased. Accordingly, in the case where the projecting optical system further includes a reflecting optical system such as a concave mirror, and light (i.e. linearly polarized light) of the respective colors transmitted through a lens group (i.e. a refracting optical system) is enlarged and projected onto the projection screen by the concave mirror or a like device, the intensity distribution of light may be made more non-uniform resulting from a non-uniformity in the light reflectance with respect to the concave mirror.
The above drawback has been described by taking an example that three liquid crystal panels are used as imagers. The above drawback, however, may also occur in an arrangement that a single transmissive liquid crystal panel is used as an imager, or a like occasion. In other words, the above drawback is a drawback which may equally occur in the case where light of the respective colors is incident onto a refracting optical system (i.e. a lens group) or a reflecting optical system (i.e. a concave mirror) as linearly polarized light which is oscillated in one direction.
In order to eliminate the above drawback, there may be proposed an arrangement, for instance, wherein a quarter wavelength plate is arranged between a dichroic prism and a projecting optical system to allow light of the respective colors to be incident onto the projecting optical system in a state of circularly polarized light or elliptically polarized light. The above arrangement eliminates likelihood that the polarization state of light may be changed depending on the incident position, thereby enabling to suppress non-uniformity in the light transmittance or the light reflectance resulting from a change in the polarization state. As a result, the intensity distribution of light transmitted through the projecting optical system can be made substantially uniform.
However, if light is incident onto a projection lens in a state of circularly polarized light or elliptically polarized light, the light transmittance may be made substantially equal to about an average of light transmittances to be obtained in the case where the light is incident as P-polarized light and in the case where the light is incident as S-polarized light. Similarly, the light reflectance to be obtained in the case where the light is incident onto a concave mirror as circularly polarized light or elliptically polarized light may be made substantially equal to about an average of light reflectances to be obtained in the case where the light is incident as S-polarized light and in the case where the light is incident as P-polarized light.
Accordingly, if a quarter wavelength plate is arranged in the aforementioned manner, the light transmittance with respect to the projection lens, and the light reflectance with respect to the concave mirror may be considerably decreased, as compared with the light transmittance to be obtained in the case where light is incident onto the projection lens as P-polarized light, and the light reflectance to be obtained in the case where light is incident onto the concave mirror as S-polarized light, respectively. In other words, the above arrangement may lower light use efficiency, and impair high luminance performance of a projector, despite an advantage that the uniformity in light intensity distribution is improved.